The present invention relates generally to the communication of data upon a channel susceptible to fading, such as a radio channel upon which data is transmitted during operation of a cellular communication system. More particularly, the present invention relates to apparatus, and an associated method, by which to increase the transmission diversity of the data communicated upon the channel, thereby to facilitate the recovery of the data once received at a receiving station.
A variant of a TCM (Trellis Coded Modulation) scheme is provided which provides for both spatial and time redundancy when used at a multiple antenna transmitter.
The use of wireless communication systems has achieved wide popularity in recent years as a result of advancements in communication technologies. Multi-user, wireless communication systems of improved capabilities are regularly utilized by large numbers of consumers to communicate both voice and nonvoice information.
In a wireless communication system, a communication channel formed between a sending station and a receiving station is a radio channel defined upon a portion of the electromagnetic spectrum. Because a radio channel forms a communication link between the sending and receiving stations, a wireline connection is not required to be formed between the sending and receiving stations to permit the communication of data between the stations. Communication by way of a wireless communication system is thereby permitted at, and between, locations at which the formation of a wireline connection would not be practical. Also, because a communication channel is formed of a radio channel, a radio communication system can be more economically installed as the infrastructure costs associated with a wireline communication system are significantly reduced.
A cellular communication system is exemplary of a wireless, multi-user radio communication system which has achieved wide levels of usage and which has been made possible due to advancements in communication technologies. A cellular communication system is typically formed of a plurality of fixed-site base stations installed throughout a geographical area which are coupled to a PSTN (Public-Switched, Telephonic Network). Portable transceivers, typically referred to as mobile stations, mobile terminals, or cellular phones, communicate with the base stations by way of radio links.
A cellular communication system efficiently utilizes the portion of the electromagnetic spectrum allocated thereto. Because of the spaced-apart positioning of the base stations, only relatively low-power signals are required to effectuate communications between a base station and a mobile station. As a result, the same frequencies can be reused at different locations throughout the geographical area. Thereby, communications can be effectuated between more than one set of sending and receiving stations concurrently at separate locations throughout the area encompassed by the cellular communication system.
In an ideal communication system, a communication signal, when received at a receiving station, is substantially identical to the corresponding communication signal when transmitted by a sending station. However, in a non-ideal communication system in which the communication signal must be transmitted upon a non-ideal communication channel, the signal, when received at the receiving station, is dissimilar to the corresponding communication signal when sent by the sending station. Distortion of the communication signal caused by transmission of the communication signal upon the communication channel causes such dissimilarities to result. If the distortion is significant, the informational content of the signal cannot be recovered at the receiving station.
The communication channel might be of characteristics which distort the value of the information bearing bits conveyed by a communication signal. Fading, such as that caused by multi-path propagation, or Rayleigh fading, alters the communication signal during its transmission. Such distortion, if not corrected, reduces the communication quality levels in a communication session formed between a sending and receiving station.
Various techniques are utilized to overcome distortion introduced upon a communication signal as a result of transmission upon a non-ideal communication channel.
Time encoding of the communication signal, prior to its transmission, for instance, increases the redundancy of the transmitted signal. By increasing the time redundancy of the signal, the likelihood of the informational content of the signal being recoverable, once received at the receiving station, is increased. Increasing the time redundancy of the signal is sometimes referred to as creating time diversity.
Also, space diversity is sometimes also utilized, for transmission of communication signals. Typically, space diversity refers to the utilization of more than one transmit antenna transducers from which a communication signal is transmitted, thereby to provide spatial redundancy. The two antennas must be separated enough to insure that their signals fade in an uncorrelated fashion. The use of space diversity does not have to be separated from encoding in the time domain. When space and time diversity are used together, the encoding in the time domain should be done jointly, across the different antenna transducers, in order to efficiently combine the two forms of diversity.
Combinations of both space and time coding further enhances transmission diversity to combat signal fading caused by multi-path transmission. At any symbol epoch, exactly one symbol is transmitted from each transmit antenna. Each transmitted symbol is selected from the constellation of signal points that characterizes the modulator associated with a particular antenna. Note that the constellations pertaining to the different transmit antennas can be in general different, but in practice it may be preferable to have identical signal constellations for all transmit antennas. The particular constellation points selected to be sent over the different transmit antennas during an arbitrary (multiple) transmission are appropriately determined from the encoder""s output symbols. Trellis encoding is sometimes used to effectuate space time coding. But, block coding is valid, too. In the former case, the selection of the constellation points, starting from the encoder""s output symbols, is decided by a construction, referred to as a trellis, which describes all possible transitions between a given, finite number of states. The states are tuples of certain most recent symbols, e.g., bits, applied to the input of the trellis encoder. For example, if the input sequence consists of raw information bits, then the tuples reflect the most recent past history of the information bit sequence which is provided to the trellis encoder, and the trellis describes a transformation of an input sequence of bits, into an output sequence of symbols, referred to as a coded symbol sequence. Note that the coded symbols can be nonbinary, too. The trellis is represented by successive columns, referred to as states, and transitions between states of successive columns are referred to as transitions. Each branch corresponds to a particular combination of new input symbols while in a given state. A mapper is utilized to map each coded symbol to a signal constellation point, thus determining the modulation parameters for a carrier signal.
In construction of the trellis and the mapper, a significant goal is to optimize the manner by which labels to trellis branches are assigned and to optimize the manner by which constellation points are assigned to the symbols used in the trellis branch labels. The optimality of the assignation is characterized in terms of a measure, referred to as a distance between two different codewords. The distance, ultimately, is determinative of the physically-meaningful, probability of a receiving station mistaking one codeword for another. The smaller the probability of a mistake, the better shall be the performance of a space-time code that is utilized in the effectuation of the communication. In order to ensure as large of a distance as possible between two codewords, a succession of points selected, during transmission, from the signal constellation, as dictated by the trellis, must be carefully determined during initial construction of the trellis. One approach to doing this is to maximize the minimum among all possible distances between pairs of transmitted codewords. To do this, codes are selected whose trellises have as large as possible pair wise distances between codewords. But, the distance spectrum is important too; it may be acceptable to allow a smaller minimum distance if that distance occurs very seldomly.
A set of all signals that possibly can be selected for transmission upon a multiple number of transmit antennas, within a meaningful time interval and according to all possible patterns of input symbols, forms a space-time code. Subsequent to constructing the space-time code, the space-time code is implemented as an encoder at a sending station and as a decoder at a receiving station. A significant problem is to determine a manner by which to efficiently select points from a given signal constellation, in such a manner as to optimize an overall performance of the transmission scheme. Performance is defined, for instance, in terms of a Bit Error Probability (BEP).
Conventionally, however, space-time coding is not optimized. This is because incorrect distance measures are utilized in trellis construction, in order to tentatively optimize the codeword separation by properly selecting points from the signal constellations used by the modulators associated with all of the transmit antennas. Note that these constellations can be in general different but in practice it may be preferable to have identical signal constellations for all transmit antennas. Conventionally, a metric suitable for space-time code design is not properly identified. Typically, a distance measure is selected based upon convenience and result in incomplete criteria of code design. The determinant criterion and the maximum rank criterion are conventionally the criteria utilized in space time code design. The former is nonconstructive and the latter only copes with the achievable level of transmit diversity.
Finally, note that while the selection of points from the signal constellations of the transmit antennas can be done via a trellis, this is not the only possibility.
If a manner could be provided by which to better jointly encode across the multiple transmit antennas, improved communication quality in a communication system in which data must be communicated upon a channel susceptible to fading would result.
Traditionally, TCM has been devised and tentatively perfected for systems using one transmit antenna. This is an important differentiator from the multiple antennas needed when coding with both space and time redundancy. Note that, in general, TCM schemes designed for Gaussian channels perform poorly in fading; likewise, trellis codes designed specifically for fading channels fail to perform well in Gaussian channels. In an attempt to deal with trellis codes for fading channels, people even considered doing away with the TCM concept.
As opposed to the one transmit antenna case, the presence of more than one transmit antenna allows for diversity even in Rayleigh fading (flat fading for each individual antenna). This should change the approach decoding because, as diversity is taken advantage of at the receiver, the fading is smoothed out and the resulting signal behaves more like having passed through a Gaussian channel. Note that this was not the case with one transmit antenna and flat fading, in the absence of other sources of diversity. Therefore, when coding with both space and time redundancy, it is desirable to have a code design that performs well not only in fading but also in Gaussian channels. The simple presence of spatial diversity changes the code design problem considerably, a fact that has not been taken into consideration so far.
It is in light of this background information related to communication of data upon a channel susceptible to fading that the significant improvements of the present invention have evolved.
The present invention, accordingly, advantageously provides apparatus, and an associated method, by which to increase the transmission diversity of data communicated upon a communication channel susceptible to fading. By increasing the transmission diversity of the data, recovery of the data, once received at a receiving station, is facilitated.
In one aspect of the present invention, a modulator is provided for a sending station operable to send a communication signal representative of the data to be communicated. The modulator is constructed in such a manner as to constrain a sequence of transmitted constellation points to behave in a desired fashion. Through operation of an embodiment of the present invention, optimal space-time codes are determinable, when optimal space-time codes exist. And, if optimal space-time codes do not exist, a suboptimal space-time code is determinable. Through such determination, the receiving station is best able to recover the informational content of a communication signal received thereat.
In operation, a natural distance measure is first identified between two codewords and thereafter is used to characterize the set of all codeword difference matrices responsive thereto. For any arbitrary pair of codewords e and c, a codeword difference matrix Dec is formed. The difference matrix is formed by performing a component-wise difference between the two codewords and arranging the results in rows and columns; for example, the columns correspond to the transmit antennas and the rows to the time epochs. In an embodiment in which two antenna transducers are used to provide space diversity, the results are arranged in two columns, one column for each of the antenna transducers. And, the distance criterion used to characterize the set of all codeword difference matrices maximizes the minimum distance between any two codewords amongst all of the possible pairs of codewords. The natural definition of the square of the distance between any two codewords is the sum of all eigenvalues of a square matrix formed of the product of the Hermitian of a code difference matrix multiplied together with the code difference matrix. The result is a true distance measure of Euclidean type that verifies all of the axioms of a metric. Additionally, the distance squared exhibits an additive property which permits simplification of the computation of a distance spectrum. And, the pairwise error probability (PEP) of any two codewords is maximized if and only if the resultant product matrix is diagonal and its eigenvalues are equal and as large as possible. That is to say, the PEP between the codewords is maximized if the resultant product matrix is diagonal and all diagonal elements of the resultant product matrix are equal. The codes that satisfy these criteria are optimal, as they achieve minimal pairwise (codeword) error probability on the average, and thereby lower the possible average BEP. If these conditions are impossible to be met for a particular signal constellation and a particular number of transmit antennas, the best suboptimal codes are those for which the resultant product matrix is as close to diagonal as possible and the diagonal element of such metric are as close to each other in value as possible. This condition is more general and applies in more general contexts than modulators whose signal points are taken from the complex field. E.g., the modulator could be an algebra of dimensionality higher than 2.
In one implementation, a modulator is provided for a transmitter portion of a mobile station operable in a cellular communication system. Analogous structure also forms portions of transmitter circuitry of a base station operable in a cellular communication system. The modulator is operable to map encoder output symbols applied to the modulator into modulator output symbols wherein the modulator output symbols form codewords. When a code difference matrix, formed between any arbitrary two codematrices, is multiplied on the left by its Hermitian, the product matrix is diagonal and all its diagonal elements are equal, thereby ensuring that the distance between any two codewords is made as large as possible.
In another aspect of the present invention, a TCM (Trellis Coded Modulation) scheme is provided for a multiple antenna transmitter. The TCM scheme provides a novel manner by which to impart optimality to the coding used to provide both spatial and time redundancy thereby extending the classical TCM scheme to form what is referred to herein as EDTCM (Enhanced modulator Dimensionality Trellis Coded Modulation). The EDTCM scheme provides good spectral efficiency by accommodating transmit diversity by way of multiple transmitted antennas and behaves optimally in both fading and AWGN (Average White Gaussian Noise) channels, while simultaneously achieving full transmit diversity and the maximum achievable modulator rate thereby.
In one implementation, the individual antenna constellations are combined, used on each of the multiple antennas are combined into one overall, equivalent, super-constellation of points. The resultant super-constellation achieves both full transmit diversity and the largest achievable modulator rate thereof. Optimal use of transmit diversity by implicit diversity-combining at a receiving station is also facilitated by the scheme under discussion. A piece-wise construction is performed over some fixed number, less than the frame length, of consecutive channel signal epochs at a time, and then extended in a piece-wise manner to an entire frame, while preserving optimality in both fading and AWGN channels. The piece-wise construction also permits handling of fast fading conditions. The admissible coherence time is the maximum between the number of consecutive epochs and the estimation time of a channel estimator. By enhancing the modulator dimensionality through the use of a super-constellation, the distance properties of the modulator are improved.
In these and other aspects, therefore, a modulator, and an associated method, is provided for a sending station operable in a radio communication system in which the sending station is operable to send data upon a communication channel susceptible to fading to a receiving station. The modulator includes a mapper coupled to receive a group of encoder output symbols in which the encoder output symbols are encoded representations of the data to be communicated upon the communication channel. The mapper maps the group of encoder output symbols to at least a first sequence of modulator input symbols. The at least first sequence of modulator output symbols is formed of at least one symbol. And, the at least first sequence of modulator output symbols forms a codeword such that a Hermitian of a difference matrix formed between an arbitrary pair of codewords multiplied by the difference matrix forms a diagonal product matrix having all diagonal elements equal to each other.
In these and other aspects, also therefore, a method, and an associated apparatus, forms a codeword which, when transmitted upon a communications channel makes efficient use of (1.) the forms of diversity present in the system, through space and time redundancy and (2.) of the noise statistics. A super-constellation of points is formed. The points of the super-constellation are selected to exhibit, when assembled together to form a codeword and transmitted upon the channel, a selected level of space diversity. The points of the superconstellation when assembled together to form a codeword and transmitted upon the channel, are preferably also selected to maximize the product distance (PD). The PD is the product of eigenvalues of Dec HDec. The super-constellation is partitioned into at least two subsets of points. The points of each subset into which the super-constellation is partitioned are selected to be maximally spaced with respect to Euclidean distance.
A more complete appreciation of the present invention and the scope thereof can be obtained from the accompanying drawings which are briefly summarized below, the following detailed description of the presently-preferred embodiments of the invention, and the appended claims.